Indoline-2-carboxylic acids have heretofore been prepared by first forming the corresponding indole-2-carboxylic acid and then reducing the 2,3-double bond. The chief drawback in such preparations has been the lack of a satisfactory process for reducing the 2,3-double bond.
One of the methods of obtaining the indole-2-carboxylic acid to be reduced has been via the Reissert synthesis wherein o-nitrophenylpyruvic acid (II above where X is hydrogen) is reductively cyclized directly to the indoline-2-carboxylic acid using zinc and acetic acid or ferrous sulfate and ammonium hydroxide. See Weissberger, ed., The Chemistry of Heterocyclic Compounds, Vol. 25, part 1, pp. 396-399 (Wiley Interscience, New York, 1972).
Three methods are reported for reducing indole-2-carboxylic acid to indoline-2-carboxylic acid. Hudson and Robertson, Australian Journal of Chemistry, 20, 1935-41 (1967), first converted the acid to the amide and reduced the 2,3-double bond of the amide using phosphonium iodide/hydriodic acid. They then converted the resulting indoline-2-carboxamide to the desired indoline-2-carboxylic acid by hydrolysis. Y. Omote et al., Nippon Kagaku Zasshi, 87, 760 (1966), also reported an indirect reduction. [See Stanton et al., Journal of Medicinal Chemistry, 26, 1267-77 at 1268 (1983)]. In this method the indole-2-carboxylic acid was first converted to the N-acetyl derivative which was reduced by hydrogenation at atmospheric pressure in the presence of platinum oxide. The resulting N-acetyl-indoline-2-carboxylic acid was then hydrolized to remove the acetyl group. Corey et al., Journal of the American Chemical Society, 92, 2476-2488, at 2480 (1970), directly reduced indole-2-carboxylic acid ethyl ester using excess dry hydrogen chloride gas and tin and absolute ethanol in a sealed bomb.
Applicants' new process for producing indoline-2-carboxylic acids avoids these indirect and inefficient reductions by cyclization of their new intermediate .alpha.-chloro-2-aminobenzenepropanoic acids V to yield the desired indoline-2-carboxylic acids directly.
The indoline-2-carboxylic acids of Formula I are useful as starting materials for the preparation of N-(3-mercapto-2-alkyl-1-oxopropyl)-indoline-2-carboxylic acids and N-(2-substituted-1-oxoalkyl)-indoline-2-carboxylic acids thereof which have antihypertensive and angiotensin converting enzyme (ACE) inhibitory properties. These antihypertensive agents and ACE inhibitors are disclosed, respectively, in U.S. Pat. No. 4,303,583, issued on Dec. 1, 1981, to D. H. Kim and R. J. McCaully and in U.S. Pat. No. 4,350,633, issued on Sept. 21, 1982, also to D. H. Kim and R. J. McCaully.